System and method for precise, accurate and stable optical timing information definition

ABSTRACT

An optoelectronic timing system includes an adaptive frequency generator system in which optical pulses are developed by a semiconductor laser. The pulses are directed into a number of time-quantifiable optical paths. Time quantification for a pulse is based upon the time required for a pulse to travel a particular length at the speed of light. Pulses are recombined at a nodal point and exhibit a numerical relationship with the periodicity of the issued pulse train equal to the numerical relationship between the lengths of the number of optical waveguides. A pulse detector and a regenerator are coupled to the semiconductor laser. A regeneration waveguide having a length equal to the longest of the optical paths is coupled to receive pulses from the laser. A pulse traveling the regeneration waveguide and directed to the pulse detector and regenerator triggers the laser to issue a next pulse, the physical length of the regeneration waveguide defining a fundamental frequency of the system and the number and lengths of the optical paths defining multiples of the fundamental frequency.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is related to and takes priority fromU.S. Provisional Patent Application entitled “System And Method ForPrecise, Accurate And Stable Optical Timing Information Definition”,Serial No. 60/434,539, filed Dec. 18, 2002, and is further related toco-pending U.S. patent applications entitled “System And Method ForDeveloping High Output Power Nanosecond Range Pulses From ContinuousWave Semiconductor Laser Systems” and “System And Method For Precise,Accurate And Stable Optical Timing Information Definition IncludingInternally Self-Consistent Substantially Jitter Free Timing Reference”,all commonly owned by the assignee of the present invention, the entirecontents of which are expressly incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention is directed generally to optical timingsystems and, more particularly to systems and methods for generation andprocessing of high speed native timing signals in the gigahertz region.

BACKGROUND OF THE INVENTION

[0003] Internationally, telecommunications is undergoing major rapidchange brought about by regulatory changes, increasingly open marketsand technological advances in integrated circuits, optical devices, andcomputer systems. The convergence and the integration of thesetechnologies, coupled with the driving factors of faster transmissionspeeds, lower signal levels, and denser circuit boards has made managingsignals in electronic and communication switching systems critical.

[0004] These driving factors have placed a greater emphasis on managingproblems relating to signal integrity, timing distribution, timingjitter, signal distribution, noise, asynchronism, and cross-talk. Inlong-haul transmission domain optical amplifiers, together withwavelength-division multiplexed have revolutionised high speed datatransmission by providing flexible and cost-effective means ofamplifying and processing of signals almost entirely in the opticaldomain independent of data rate and protocols.

[0005] Impairments suffered by timing signals play a critical role inelectronic systems. They limit the dynamic range of ananalogue-to-digital converter, the throughput of a digital bus, affectthe behaviour of digital synchronisers, influence the bit error ratio ofa communications link, and determine the sensitivity and selectivity ofradio receivers. Timing impairments are the result of random noise andsystematic disturbances within electronic devices and interconnections.

[0006] Electronically derived timing signals suffer from an additionalinherency constraint; the limits of electronic system frequency responseis predicated on the internal parasitic capacitances developed as anartifact of the functional underpinnings of active semiconductorintegrated circuit devices. The familiar P-N junction which forms thebasis of active device fabrication, whether expressed in terms ofmajority or minority carrier devices, nevertheless inherently compels aparasitic capacitance be coupled into the elemental circuit model.

[0007] Thus, in the rapidly advancing telecommunications field forexample, electronically generated timing signals are becomingincreasingly problematic as fundamental limits of integrated circuitfrequency response are reached. However, the burgeoning field ofoptoelectronics offers a means of avoiding a strict dependence onelectrical/electronic timing signal generation. Optoelectronics ispredicated upon optical signal processing and inherently includestechnologically satisfactory structures for confining and transmittingoptical pulses over great distances. Since the speed of light has a wellrecognized constant value, given a particular transmission medium, alight pulse can be utilized to define a non-relative andnon-relativistic methodology for measuring time as well as timeintervals. A light pulse traveling at a constant velocity, traversing aknown distance, in the same reference frame as an observer, provides asimple and inherently stable method for defining a time interval.Mechanical definition of a multiplicity of branching travel paths offersa straight forward way of constructing a timing generator characterizedby timing trigger edges having native periodicities in the gigahertz andmulti-gigahertz regime.

SUMMARY OF THE INVENTION

[0008] In an optoelectronic timing system, an adaptive frequencygenerator system comprises at least one semiconductor laser configuredto issue subnanosecond optical pulses defining a periodic pulse train.At least a first optical waveguide is configured to define a firsttime-quantifiable optical path for a pulse of the train and at least oneadditional optical waveguide is configured to define a secondtime-quantifiable optical path for a pulse of the train different fromthe first waveguide.

[0009] Pulses of the train are directed into the first and secondwaveguides at a first nodal point coupled to the first and secondwaveguides and pulses directed into the first and second waveguides arerecombined at a second nodal point coupled to the first and secondwaveguides. The length of the second time-quantifiable optical path hasa defined numerical relationship to the length of the firsttime-quantifiable optical path, such that the periodicity of pulsesrecombined at the second nodal point has the same numerical relationshipwith the periodicity of the issued pulse train.

[0010] In one aspect of the invention, the at least one semiconductorlaser is configured to provide a pulsed output having a periodicity inthe range of about 1 nanosecond so as to define a 1 gigahertz pulsetrain. Additionally the second optical time-quantifiable optical pathhas a length differing from the first time-quantifiable optical path byabout 0.5 nanoseconds, so as to define a 2 gigahertz pulse train at thesecond nodal point.

[0011] One feature of the present invention allows expansion of thenumber of time-quantifiable optical paths to provide for adaptivelyshorter periodicities. In this aspect, the invention comprises amultiplicity of additional optical waveguides each coupled to the firstand second nodal points, the additional waveguides configured to definea multiplicity of time-quantifiable optical paths. The lengths of eachof the multiplicity of additional time-quantifiable optical paths havinga numerical relationship with each other and with the firsttime-quantifiable optical path. The semiconductor laser is configured toprovide a pulsed output at a first periodicity and wherein therecombined pulse train at the second nodal point provides a pulse trainhaving a second periodicity, the second periodicity being a multiple ofthe first, the multiple defined by the numerical relationship betweenthe multiplicity of additional time-quantifiable optical paths and thefirst time-quantifiable optical path.

[0012] As an example, the semiconductor laser operates at a frequency ofabout 1 gigahertz and the lengths of the multiplicity oftime-quantifiable optical paths differ from one another by about 0.2nanoseconds, so as to define a 5 gigahertz pulse train at the secondnodal point. Characteristically, time quantification of the optical pathlength is defined by the distance required for a pulse to travel at thespeed of light for a given time interval.

[0013] 17. In an optoelectronic timing system, an adaptive frequencygenerator system comprising:

[0014] at least one semiconductor laser configured to issuesubnanosecond optical pulses defining a periodic pulse train;

[0015] a multiplicity of optical waveguides, the waveguides configuredto have physical lengths differing from one another by a numericalrelationship, each length defining a time-quantifiable optical path fora pulse of the train based upon the time required for a pulse to travela particular length at the speed of light;

[0016] a first nodal point coupled to the multiplicity of opticalwaveguides at which pulses of the train are directed into themultiplicity of optical waveguides;

[0017] a second nodal point coupled to the multiplicity of opticalwaveguides at which pulses directed into the multiplicity of opticalwaveguides are recombined; and

[0018] wherein, the periodicity of pulses recombined at the second nodalpoint has the same numerical relationship with the periodicity of theissued pulse train as the numerical relationship of the multiplicity ofoptical waveguides.

[0019] In a further aspect, the invention comprises a pulse detector anda regenerator coupled to the pulse detector and semiconductor laser. Aregeneration waveguide having a length equal to the longest length ofthe multiplicity is coupled to receive pulses from the laser. Theregeneration waveguide is not coupled to the first or second nodalpoints. A pulse traveling the regeneration waveguide is directed to thepulse detector and regenerator so as to trigger the laser to issue anext pulse. The physical length of the regeneration waveguide defines afundamental frequency of the system.

[0020] The periodicity of pulses recombined at the second nodal pointaccordingly defines a frequency which is a multiple of the fundamentalfrequency of the system, the numerical value of the multiple being equalto the number of the multiplicity of optical waveguides. As an example,were the fundamental frequency of the system to be defined as 1gigahertz, i.e., 1 nanosecond, five (5) waveguides differing from oneanother by 0.2 nanoseconds would define a five (5) gigahertz pulsetrain. Lengths of the wave guides are defined in accordance with anumerical relationship based on the distance required for an opticalpulse to traverse in one (1) nanosecond while traveling at the speed oflight in the medium defining the wave guide.

[0021] Waveguides may be disposed in semiconductor material, provided asoptical fiber, doped or undoped, or provided as a free-space path ineither a vacuum or a gaseous ambient.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] These and other features, aspects and advantages of the presentinvention will be more fully understood when considered with respect tothe following specification, appended claims and accompanying drawings,wherein:

[0023]FIG. 1 is a simplified, semi-schematic output trace of laser pulseamplitude and pulse width as a function of input pulse amplitude andpower;

[0024]FIG. 2 is a simplified, semi-schematic structural diagram of oneembodiment of a laser retriggering circuit in accordance with theinvention;

[0025]FIG. 3 is a simplified, semi-schematic structural diagram of asecond embodiment of a laser retriggering circuit in accordance with theinvention;

[0026]FIG. 4 is a simplified, semi-schematic diagram of one embodimentof an optical timing system in accordance with the invention;

[0027]FIG. 5 is a simplified, semi-schematic diagram of a secondembodiment of an optical timing system in accordance with the invention;

[0028]FIG. 6 is a simplified, semi-schematic diagram of an additionalembodiment of an optical timing system in accordance with the invention,including a tiered configuration;

[0029]FIG. 7 is a simplified, semi-schematic diagram of one embodimentof a precision optical timing system in accordance with the invention;

[0030]FIG. 8A is a simplified, semi-schematic diagram of one embodimentof a pulse delay optical compensator circuit in accordance with theinvention;

[0031]FIG. 8B is a simplified, semi-schematic diagram of one embodimentof a pulse advance optical compensator circuit in accordance with theinvention;

[0032]FIG. 8C is a simplified, semi-schematic diagram of one embodimentof a combination pulse delay and pulse advance optical compensatorcircuit in accordance with the invention;

[0033]FIG. 9 is a simplified, semi-schematic diagram of one embodimentof an adaptive pulse phase compensator circuit in accordance with theinvention;

[0034]FIG. 10 is a simplified, semi-schematic circuit diagram of oneembodiment of a logical gate implementation for pulse delay and advancein an optical compensator circuit in accordance with the embodiment ofFIG. 9.

DESCRIPTION OF THE INVENTION

[0035] Continuous wave laser diodes are well known in the semiconductorlaser arts. They provide a low cost and physically small solution fordevelopment of optical and optoelectronic systems. While very useful inoptical data transmission applications, continuous wave lasers havesignificant disadvantages when used in optical timing applications, notthe least of which is their conventional inability to deliver a pulsedoutput signal with a sufficiently high output power. This is quitedisadvantageous when a continuous wave laser is coupled to an opticaltransmission path such as a waveguide or optical fiber.

[0036] Optical transmission paths are known to attenuate laser signalenergy as well as disperse the output waveform (a process termed pulsespreading) thereby requiring periodic electronic amplification and pulsesquaring circuitry to be provided in the signal path. Where the initialoutput signal is relatively weak, such amplification and shapeprocessing must be provided more frequently, resulting in expensive andhighly complex installations. Accordingly, although able to be operatedin a pulse mode, continuous wave semiconductor lasers have not beenconsidered for timing applications because of what is conventionallyconsidered their inherently limited output power characteristics.

[0037] It has been determined, however, that virtually all continuouswave laser diodes are able to be operated in a certain manner in orderto achieve a high power pulsed output by operationally exercising themusing a sub nanosecond input pulse having an IV (power) characteristicat or exceeding a particular derived current-voltage (IV) thresholddescribed in more detail below. The operation of continuous wave laserdiodes in this manner is not described in any manufacturer's data sheetsnor are they currently known by those having skill in the art. Indeed,the particular current-voltage thresholds required to operate theselasers in a high output power pulse mode is far above the manufacturer'soptimum operating levels, and indeed far exceeds the nominal inputthreshold for any semiconductor laser diode examined.

[0038] Although the input characteristics are far in excess of nominaltolerance levels, this is not considered to pose any operationaldangers, because the input pulse is defined as having a duration of lessthan about one nanosecond and a duty cycle of about less than a 25%.Consequently, there is not enough time for heat accumulation to occurand therefore no thermal damage is imparted to the laser material. Theuse of continuous wave lasers in this manner is not isolated to anyparticular laser diode composition nor is it limited to lasers operatingwithin any particular wavelength regime. The operational characteristicsof the present invention have been demonstrated on various lasercompositions including AlGaIn, AlGaInP, GaAlAs, AlGaAs, and the like,having wavelengths of from about 600 nanometers to about 1600nanometers.

[0039] Even for those few continuous wave laser diodes that do have apulse mode defined by the manufacturer, the stated output for thesediodes is typically far below what one is able to achieve for a pulsedoutput in accord with the invention. For example, the Hitachi HL7581Gcan be pulsed per the manufacturer's specifications at approximately 2Vbut is only capable of achieving an output pulse with a powercharacteristic of from about 50 to about 60 milliwats. A continuous wavelaser operated in accordance with the present invention is able todefine a pulsed output exhibiting a considerably higher output power; inthe case of the Hitachi HL7581G, operating the laser diode in accordancewith the invention will allow one to achieve a 3000 milliwatt output,i.e., about two orders of magnitude higher output power than previouslycontemplated.

[0040] Specifically, it should be understood that the majority of thesemiconductor continuous wave lasers are designed to operate with aninput voltage of from about 2.5 V to about 3 V and require an inputcurrent of from about 50 to about 150 milliamps, for continuous waveoperation. In the case of the Hitachi HL7581G, mentioned previously, thespecified input threshold current is about 45 milliamps (at 25 C) andthe output power, as a function of injection current, is specified tofollow an approximately 0.5 milliwatt per milliamp slope (conventionallytermed a 0.5 mW/mA slope efficiency). Nominal operating current is about140 milliamps at the nominal 50 mW output.

[0041] Where a pulse mode is explicitly stated, the operationalcharacteristics are such that they remain generally within the specifiedinput and output constraints of continuous wave operation. In theHitachi HL7581G case, pulsed operation is allowed with a pulsed opticaloutput power of about 60 mW, a 50% maximum duty cycle and a maximumpulse width of about 1 microsecond. Typically, the maximum continuouswave output of conventionally operated continuous wave devices rangesfrom about 5 to about 70 milliwatts, for typical laser diodes havingnominal input impedances of generally less than 1 Ohm (typically in therange of about 0.2 Ohms). When the maximum specified injection current(I) is considered for a range of continuous wave laser diodes, alongwith their specified maximum operating voltage (V), is it relativelysimple to derive a range of corresponding operational input powercharacteristics (IV=W) for these lasers of from about 0.025 Watts toabout 0.35 Watts.

[0042] In accordance with the present invention, a continuous wave lasermay be operated at a particular input regime, characterized by aparticular operational input power characteristic (a current-voltagethreshold termed herein the Siepmann Threshold or ST) in order to obtaina high output power pulse in the subnanosecond range. This threshold(ST) is experimentally determined for each laser diode, and typicallylies in the range of from about 2.0 to about 6.2 Watts, depending on theparticular laser diode composition and construction under investigation.Once the ST threshold has been determined for each laser diode, thatlaser may be operated to obtain output pulses having similar powercharacteristics, i.e., output pulses in the range of about 3000milliwatts. Although not particularly relevant to practice of theinvention, it has been observed that the Siepmann Threshold appears tohave a direct proportionality relationship to the surface area of thelaser diode at issue, all other factors (such as composition andwavelength) being equal.

[0043] In particular, the lowest current-voltage threshold necessary toachieve a subnanosecond high power pulsed output from each of thecontinuous wave laser diodes evaluated, ranged on the order of fromabout 12 to about 160 times the manufacturer's maximum input powerratings for the particular laser diode at issue. The Siepmann thresholdmay be obtained for virtually any continuous wave laser diode, with themaximum output of each diode being found somewhere in the range of fromabout one to about two times the ST threshold for that diode.

[0044] Operationally, and in accordance with principles of theinvention, the Siepmann Threshold (ST) is found by applying asubstantially increased injection current to the device at an increasingoperating voltage (Vop) and evaluating the device's outputcharacteristic. The current injection is provided in pulse fashion andis generally in the range of from about 200 picoseconds to about 600picoseconds, but it could be substantially less. The pulse width should,however, be maintained in the range of less than 1 nanosecond. In orderto avoid damage to the diode, it will be necessary to maintain the inputpulse at a duty cycle of less than about 25%, and preferably less thanabout 20%. Input current and operating voltage are increased until thedevice's output characteristic exhibits a substantial and quitesurprising jump in measured output power. Notably, the output powerincrease is not linear. Output power remains generally within specifiedtolerances until the ST threshold is reached for each diode. At the STthreshold, however, the output power characteristic jumps at least oneorder of magnitude and typically two orders of magnitude.

[0045] It should further be mentioned that the ST threshold may beconveniently found by starting the procedure utilizing the laser diodemanufacturer's rated maximum operating voltage and injecting anoperating current in the range of about 1 Amp. From this starting point,one having skill in the art can easily determine a set ofcurrent-voltage matrix values that will identify the point at which theST threshold determines operation of the device. Current may be swept,with voltage incrementally stepped for each sweep, or vice versa.Alternatively, a set of IV “corner” values may be generated and IVsweeping performed about the corners in accord with well understoodprinciples of experimental statistical data analysis.

[0046] The minimum target input power is about 2.0 Watts, with severaldevices exhibiting ST thresholds in the range of about 6 Watts. Notably,it would appear that increases in the operating voltage have a morebeneficial effect in deriving the ST threshold than increases in theinjected current. Pulsing a CW laser diode with an input pulse amplitudein the 4000 to 5000 millivolt region, while maintaining the inputcurrent in the 1 Amp region seems to be able to develop the ST thresholdfor most devices.

[0047] Turning now to FIG. 1, there is shown an exemplary outputcharacteristic curve developed for a typical continuous wave laser diodeoperated at and above its derived ST threshold. In the example of FIG.1, a high output subnanosecond pulse is developed at an observed STthreshold corresponding to approximately 4.0 Watts. Data was takenutilizing an input current of relatively constant value at approximately1.0 Amps; thus the input pulse amplitude is characterized in terms ofvoltage (i.e., an ST threshold of about 4000 mV). When so operated, aphotonic pulse develops at the threshold and exhibits an initial outputpulse amplitude characteristic in the range of about 200 mV, at a pulsewidth substantially equal to the input pulse width of about 150picoseconds. The photonic pulse amplitude increases as the input pulseamplitude increases until a maximal photonic pulse amplitude (MPPA) isdeveloped.

[0048] In the example of FIG. 1, the MPPA is observed to be in theregion of about 500 mV, or about at least two times the pulse amplitudeat the ST threshold. After the maximal photonic pulse amplitude isreached, further increases in the input pulse amplitude will actuallycause a decrease in the photonic pulse amplitude until a nadir isreached. Typically, this occurs at about an input pulse amplitude of 5%to 7% beyond the ST threshold amplitude (about 4270 mV in the example ofFIG. 1). Although a nadir in output pulse amplitude is experimentallyobserved, the output pulse amplitude value at the nadir is stillgenerally in the region of the output pulse amplitude defined at the STthreshold. It has been experimentally determined that the MPPA will notagain be reached after the nadir no matter how much the input pulseamplitude is increased; indeed, the output pulse amplitude is observedto plateau at a level somewhat below (approximately 5% to 10% below) themaximum output pulse amplitude developed at the MPPA.

[0049] The output pulse width, which remains generally stable until thepost MPPA nadir, is observed to increase as the input pulse amplitudeincreases beyond the value defining the photonic pulse nadir. Outputpulse widths are stretched from the nominal input pulse widths to abouttwice the nominal input value. The example of FIG. 1, indicates outputpulse stretching from a nominal value of about 200 picoseconds to avalue of about 400 picoseconds at an input amplitude of about 15% toabout 20% in excess of the ST threshold. Notionally, the output pulsewidth remains stable at about the input pulse width across the range ofinput amplitudes from the ST threshold to at least the nadir.

[0050] Accordingly, it will be understood that operating a continuouswave laser diode, in accordance with the invention, is able to define adevice which is capable of developing a subnanosecond pulsed output withan output power characteristic significantly larger than conventionallyoperated diodes. Those having skill in the art will understand that arelatively simple continuous wave semiconductor laser diode may now beutilized as an optical timing device due to its previously unknown highoutput power pulse characteristics. The use of continuous wave laserdiodes for subnanosecond pulsing in this manner in order to achieveextremely high outputs is not commonly known to those practiced in theart and indeed represents a highly surprising result. This novel useallows one to achieve a high pulse output with inexpensive laser diodesthat could otherwise only have been achieved with expensive pulse lasersystems costing thousands of dollars. Additionally, this allows smallsemiconductor lasers to be used in combination with semiconductorelectronic circuitry in order to manufacture small and inexpensive highspeed optoelectronic timing devices.

[0051] Turning now to the exemplary embodiment of FIG. 2, the novel highpower pulse mode laser diode described above can be adapted to provide alight-based timing/clocking device with operational characteristics thathave not been realized beforehand. In the particular embodiment of FIG.2, the invention is directed to a semiconductor laser which is able todeliver a subnanosecond output pulse having an amplitude characteristicin the range of more than 100 milliwatts at a pulse duty cycle ofanywhere from about 1% to about 25%. The semiconductor laser, indicatedat 10, is overdriven at or above the ST threshold, as described above,and is triggered by an injection current derived from a photoconductiveor capacitive trigger device 12, thereby yielding a high power laseroutput pulse that is less than one nanosecond in duration. In theillustrated embodiment the semiconductor laser is coupled in series withan optical fiber or an optical waveguide 14 which functions as thetiming path element, in a manner to be described in greater detailbelow. When the device is used in a closed-loop mode, i.e., an initialpulse, or a portion thereof, is used to trigger a next or subsequentpulse, approximately 20 percent of the laser initial output pulse is fedback to the trigger (photoconductive or capacitive trigger) 14 where itcauses an additional laser pulse to be generated and propagated into thewaveguide.

[0052] As was described previously, the wavelength of the laser pulse,whether initial or subsequent, is not as crucial to timing operations asthe stable functioning of the semiconductor laser itself. Accordingly,care should be taken to define the trigger output characteristics suchthat they meet the laser's determined ST threshold, and preferably thelaser's determined MPPA. Additionally it is contemplated that any outputpulse derived from the laser diode is collimated with a collimating lens16, in order to minimize dispersion of the output pulse and maximizeoptical power in each pulse propagated through the fiber or waveguide.

[0053] As shown in the exemplary embodiment of FIG. 3, thephotoconductive/capacitive layer may be further defined as comprising acapacitor-type semiconductor injection trigger 18 coupled to asemiconductor photodetector 20. When the device is used in a closed-loopmode, approximately 20 percent of the laser output pulse is fed back tothe semiconductor photodetector 20 which then sets off thecapacitor-type semiconductor injection trigger 18, in turn retriggeringthe laser 10 to develop a pulse. As was the case in the exemplaryembodiment of FIG. 2, the laser is coupled to an optical fiber orwaveguide which propagates the pulse and includes means for dividingapproximately 20% of the pulse and directing what might be termed thetrigger pulse back to the photodetector for subsequent retriggering.

[0054] Particularly suitable types of devices that might be used in thisconnection include MOS photocapacitors similar to those implemented inconventional CCD image capture and reproduction technology. It hasbecome apparent that CCD technology is able to be used in many differentpotential applications, including signal processing and imaging,particularly because of silicon's light sensitivity. Silicon responds tophotons in the optical spectrum at wavelengths less than about 1.0 mm.This is relatively important since the visible spectrum falls between0.4 mm and 0.7 mm and the majority of semiconductor CW laser diodesexhibit dominant output modes in the visible range. The CCD's superbability to detect light has turned it into the industry-standard imagesensor technology.

[0055] Further, CCD devices inherently involve a charge transfermechanism, which is easily coupled as a trigger to a high-speed chargeamplification device such as a voltage follower configured BJT or an SCRcoupled to discharge a capacitor circuit upon receipt of a nominaltrigger current value. Although the illustrated embodiments of FIGS. 2and 3 have discussed photodetectors and capacitor-type trigger devices,it will be understood that these have been discussed only for purposesof example and not to limit the construction and operation of thedevice. Photodetectors and triggers might also be thought of ascomprising initiator devices and/or regenerator devices that function tocause the laser diode to issue an appropriate pulse. Accordingly, asuitable regeneration amplifier is implemented as any form of electronicdevice operationally responsive to an optical pulse in the generallyvisible range of the spectrum, and outputting a substantially high powertrigger pulse in the subnanosecond region; such a regeneration amplifiershould be able to switch current throughout at least 500 MHz. Asmentioned above, a bipolar switch might be particularly suitable when itis understood that a high speed timing device should be able to switchthe laser operationally at speeds of about 500 MHz at current levels atabout 500 milliamps.

[0056] It should also be understood that a timing device constructed inaccordance with the invention might operate in two different modes, andopen-loop mode and a closed-loop mode. When operating in an open-loopmode, the device might be controlled by a voltage controlled oscillator(VCO) with clock speeds in excess of one gigahertz (1 GHz) beingachieved by a suitable definition of the optical path, as will bedescribed in greater detail below. In a closed-loop mode, as wasdescribed above, the trigger input for subsequent pulse development istaken from at least a portion of the initial, or some previous, pulsefrom a well defined distance along the optical path itself.

[0057] In accordance with principles of the invention, an optical timingdevice utilizes the speed of light in an ambient medium traversing adefined travel distance “d” in order to derive a stable time interval“t”. An optoelectronic device, in accordance with the invention, ispredicated upon utilization of the speed of light, with a light pulsetraversing either an optical fiber or a wave guide for a known distance,in order to define a known time. In effect, timing becomes a function ofknown distance, as opposed to a function of the vagaries of electricalor electronic switching devices.

[0058] For purposes of example, the speed of light is taken to be 3×10⁸meters per second. Although this is not precise, and also refers to thespeed of light in a vacuum, the value of 3×10⁸ meters per second issufficient for exemplary purposes. Additionally, the speed of light inother media will be understood by those having skill in the art todepend upon the index of refraction of that medium. While thesedifferences exist, they will be deemed to be not particularly relevantfor purposes of description. The actual values used in the context ofthe discussion must be recognized as exemplary only and chosen solelyfor ease of computation. Accordingly, assuming a light pulse isgenerated and propagated down an optical fiber or wave guide, thepropagation speed of such a pulse will be on the order of 30 centimetersper nanosecond of travel time (more particularly about 20 centimetersper nanosecond where the index of refraction of a typical optical fiberis on the order of about 1.5). Thus, a gigahertz timing signal mightwell be understood as comprising multiple 30 centimeter paths, with each30 centimeter path length defining the fundamental one nanosecond timinginterval.

[0059] Turning now to FIG. 4, an optoelectronic timing device, inaccordance with the invention is depicted in a simplified,semi-schematic form and is configured to define a native one gigahertztiming device operating in an open-loop mode with a 200 megahertztrigger. In the exemplary embodiment of FIG. 4, a 200 megahertz phaselock loop (PLL) 30 operates at 200 megahertz and provides a triggersignal to a semiconductor laser 32 so as to develop a high power outputpulse, in a manner described above. The laser 32 develops an outputpulse which is propagated over an optical fiber or wave guide 34 whichis substantially 150 centimeters in length and which further comprisestaps 36 at equal 30 centimeter intervals along the length of the fiberor wave guide. Each of the taps 36 defines a path to a correspondingphotodetector circuit or a centrally disposed photodetector circuit 38,so as to define a set of sequential signal sources, each source defininga timing signal one nanosecond following the previous signal source.

[0060] Since the PLL 30 is operating at 200 megahertz, the semiconductorlaser 32 fires off a new optoelectronic pulse every 200 megahertz. Eachpulse is propagated down the fiber or wave guide 34 and sequentiallycauses the photo detector 38 to receive a quantum of the optoelectronicpulse and generate an electrical signal in operational response thereto.Thus, a 200 megahertz PLL is able to define a one gigahertz nativetiming signal, without reliance upon integrated circuit electronicswitching characteristics. Further, it will be understood by thosehaving skill in the art, that since the timing properties of theresultant signal depend solely upon a known distance parameter betweenphoto detectors, the resultant timing signal will be stable andinternally self consistent and repeatable with respect to pulsesgenerated by the optical system.

[0061] In the exemplary embodiment of FIG. 4, it should be understoodthat the only timing jitter introduced to the system will result fromtiming irregularities of the 200 megahertz PLL. Since it is relativelystraight forward to accommodate for jitter and other timingabnormalities in 200 megahertz systems, an optoelectronic timing device,in accordance with the invention, will be understood to exhibit thestability and precision characteristics associated with relatively lowspeed technologies, while defining native timing pulses operating in thegigahertz regime.

[0062] It should further be understood that the exemplary embodiment inFIG. 4 has been described in terms of an open loop system with opticalpulses initiated by an external trigger device, such as a PLL.Alternatively, the apparatus can be constructed to operate in closedloop fashion by having the vestigal optical pulse retrigger thesemiconductor laser 32 at the same time that it generates an electricalpulse through the final length of the fiber or wave guide and thence tothe photodetector. In order to accommodate this function, the fiber orwave guide could be disposed in a circular arrangement or by defining afolded optical path, as is well known as those having skill in the art.Accordingly, the system should be understood as contemplating open loopas well as closed loop operation without violating the spirit of theinvention.

[0063] The exemplary embodiment of FIG. 4 was described in order todevelop an appreciation of the fundamental features of an optoelectronicdevice that operates in accordance with the invention. The system neednot be limited to a 200 megahertz trigger, nor need the system belimited to one gigahertz operation, by defining the individual waveguide segments in terms of 30 centimeters. Indeed, a 10 gigahertzoptoelectronic timing device is able to be produced with currenttechnology, by implementing the various elements in Gallium Arsenide(GaAs) integrated circuit technology rather then the more prosaic andconventional silicon. GaAs integrated circuit chips allow for inherentlyfaster operational response times because of their inherently lowerparasitic capacitances, as well as allow a semiconductor laser to befabricated in situ during the device manufacturing process. Such a lasersystem (termed a vertical cavity surface emitting laser or VCSEL) wouldnegate the need for providing and bonding a separate semiconductor laserchip onto a silicon integrated circuit. The cost savings more thencompensate for the additional manufacturing costs of GaAs integratedcircuit technology.

[0064] In order to overcome the output power limitations known to applyto VCSEL lasers, multiple VCSELs are fabricated in GaAs and directedinto a 100 micron lightguide. 10 VCSELs may easily be fabricated in GaAsto fit into the footprint required for a 100 micron lightguide. Thus, 10VCSELs can be triggered simultaneously into the wave guide in order toobviate the output power limitations of an individual laser.

[0065] From the discussion of the exemplary embodiments of FIG. 4, itwill be straight forward for one having ordinary skill in the art tounderstand that a 10 gigahertz optoelectronic timing device will have a2 nanosecond loop time for a single laser (or simultaneously initiatedset of lasers). A 2 nanosecond loop time is accomplished by utilizingappropriately devised path lengths hosting optical pulses generated by aclosed loop system, as described above, or by utilizing a 500 megahertzoscillator (PLL) to trigger the laser system. In this regard, theoptical pulse width is contemplated as being less then 200 picosecondsand, preferably, less than 100 picoseconds when the system is utilizedas a 10 gigahertz clock.

[0066] It should also be noted, that for a 2 nanosecond loop time, theoverall length of the optical fiber or wave guide which is required tosupport 2 nanosecond travel time will be on the order of 60 centimeters.

[0067] Turing now to the exemplary embodiment of FIG. 5, there is shownin simplified, semi-schematic form, a 10 gigahertz timing system,generally similar to the exemplary embodiment of FIG. 4, but comprisinga cascade laser system defining two timing portions. Each of theportions is generally similar to the embodiment of FIG. 4, with asemiconductor laser 32 being triggered by a PLL trigger device, orequivalent, and is shown as directing optical pulses down a primaryoptical fiber or wave guide 33, comprising suitably disposed tapsleading to a primary photo detector 34. However, as each pulse generatesa signal in the primary photodetector, the primary signal is seen astriggering a secondary laser, identified by the numeral 40, which, inturn, develops an optical pulse down the second portion of the system.

[0068] The second portion of the timing system in FIG. 5 is structurallyidentical to the first portion and only differs from the first portionin having it's laser triggered by each vestigal optical pulse as opposedto being triggered by an electronic timing oscillator, for example. In asense, the exemplary embodiment of FIG. 5 can be thought of a quasi openand closed loop system in combination. It will further be understoodthat the system may be implemented in a completely closed loop fashionby merely completing the circuit between the second portion of thetiming device and the pulse initiation laser 32 in a manner describedabove in connection with FIG. 4. Briefly the exemplary embodiment ofFIG. 5 is provided solely as an example of how lasers and fiber/waveguides maybe cascaded in order to define primary and secondary timingloops. In this regard, it should be understood that a 10 gigahertztiming device maybe formed utilizing several possible combinations oftrigger circuits, lasers, wave guides and taps.

[0069] For example, a 10 gigahertz timing device might comprise a singleoptical loop, utilizing a single initiation laser system and including10 outputs, each cycling through 1 nanosecond. Where the system isclosed loop or fired by a 1 gigahertz clock trigger, a simpleconstruction analysis reveals that the system operates at 10 gigahertz.Additionally, such a system might be implemented as a single unitcomprising 20 outputs each cycling at 2 nanoseconds where the laser isfired in accordance with a 500 megahertz clock trigger device.

[0070] In a two-laser system, such as depicted in the exemplaryembodiment of FIG. 6, a secondary laser 50 is nested within the primarytiming loop, such that a trigger pulse developed by the primary loops'photo detector 52 initiates an optical pulse in the secondary lasersystem 50. The primary loop cycles at 100 megahertz, by being triggeredby a 100 megahertz PLL 54, for example. The primary loop furthercomprises ten equally-spaced outputs each having a length defining a 10nanosecond interval. As a pulse traverses a particular leg of the loop,it generates a signal in the primary loops' photo detector, 52 which, inturn, fires off the secondary laser 50. The secondary laser 50 is,therefore, triggered at a 1 gigahertz rate and cycles at 1 nanosecondintervals, by defining a secondary wave guide 56, itself having tenequally-spaced apart outputs so as to define a 10 gigahertz pulse trainto the secondary photo detector 58.

[0071] Similarly, a 10 gigahertz system might be implemented as primaryand secondary units with the primary cycling at 100 megahertz andcomprising five outputs: the secondary cycling at 2 nanosecondintervals, i.e., twenty outputs, thereby defining a 10 gigahertz pulsetrain. This conceptual construction may be easily extended to athree-laser system by an additional level of nesting. A tertiary lasersystem need only be coupled into the secondary system, such that thetertiary laser is fired by the secondary photo detector electronics, andso forth. Since the actual timing rate is nothing more than anappreciation of a physical distance down a wave guide of optical fiber,it will be immediately understood by those having skill in the art thatany number of mechanical arrangements maybe implemented so as to defineoptical timing pulses having any number of desired characteristics atvirtually any technologically feasible speed. Indeed, one of theparticular utilities of the invention is it's ability to generategigahertz region timing signals utilizing very simple andstraightforward, relatively low-speed electronic trigger pulses. A 100megahertz oscillator or PLL is all that is required to initially fireoff the system into gigahertz and multiple gigahertz pulse traindefinition.

[0072] Now that the fundamental constructor in operations of an opticaltiming system, in accordance with the invention, has been described, itwould be worthwhile to discuss a few of the timing systems that can beimplemented utilizing such technology. Turning now to the exemplaryembodiment of FIG. 7, there is shown a precision serial optical timingdevice, termed herein a “lightclock”, that utilizes various aspects ofthe novel optical timing system, described above. In the exemplaryembodiment of FIG. 7, the precision serial lightclock is implemented asa set of nested timing loops, with each timing loop having a loop timeof substantially one order of magnitude smaller than the loop time ofthe proceeding. Specifically, the exemplary embodiment of FIG. 7describes six optical timing loops, nested within a main, or seventh,loop defined by implementing the system in a closed-loop fashion. Eachof the optical loops is defined by an optical fiber which, since therefractive index of optical fiber is typically 1.5, defines anapproximately 20 centimeter distance for each required nanosecond.

[0073] As described previously, a semiconductor laser 60 is fired by aninitial trigger pulse to define an optical pulse into a first timingloop 62. The first timing loop 62 is contemplated as defining a 0.01millisecond loop, thereby requiring approximately a 20 kilometer lengthof fiber, with an approximately 2 kilometer spacing between each of the10 outputs of the loop. The first optical timing loop 62 defines anoutput by having each of the fiber taps provide a signal to a centrallydisposed photo detector 64 which, in turn, provides the trigger signalto a next-level laser diode 66 which develops an optical pulse to thenext-level optical loop 68. The next-level optical loop 68 is defined asa 1.0 microsecond loop, thereby requiring an overall 2 kilometerdistance for the loop fiber, with each of 10 outputs being spacedapproximately 200 meters apart. As before, each of the outputs isdirected to a centrally disposed photo detector 70 which necessarilyreceives optical signals at one microsecond intervals. Further, thephoto detector 70 provides the necessary trigger pulse to a next-levelsemiconductor laser 72 which defines an optical pulse into thethird-level (0.1 microsecond level) optical path 74. Spacing betweenoutputs in the third-level optical path are necessarily 20 meters apartin order to define a 0.1 microsecond pulse train to the centrallydisposed photo detector 76.

[0074] Similarly, forth, fifth, and sixth optical loops (78, 80 and 82,respectively) function just as described previously, with theirrespective output spacing being 2 meters, 20 centimeters and 2centimeters, in order to ultimately provide a 100 picosecond pulse trainfrom the final timing loop.

[0075] Returning momentarily to the first-level loop of the system, itshould be noted that the 20 kilometer fiber terminates in a furtherphoto detector FIG. 4 which is used to retrigger the initialsemiconductor laser 60 in closed-loop fashion. Accordingly, the initialsemi conductor laser 60 operates at approximately 0.1 millisecondintervals. Also, each of the photo detectors (64, 70, 76, and the like)have an output coupled to a corresponding incremental counter (indicatedcollectively at 86) that offers a count-up methodology by which seconds,minutes, hours, days, etc, can be simply and easily accounted for inorder to translate the optical timing loops of the system into arigorous and extremely accurate precision time keeping device.

[0076] The system described in the exemplary embodiment of FIG. 7 can beeasily constructed by having a semiconductor-type laser pigtailed to anoptical fiber, such as SMF 28, or the laser might be a component of anoptoelectronic integrated circuit (OEIC) or a component of a printedcircuit board. Either a printed circuit board or an OEIC implementationis able to incorporate the optical fibers, photo detectors,initiation/trigger driver, and integral lasers, or any subset of thesecomponents. Additionally, a printed circuit board or OEIC implementationis able to accommodate a free space adjustment gap, between the opticalfibers and photo detectors, at any level, such that fine-tuning can beprecisely performed for path length variations. Such fine-tuning allowsmanufacturing tolerances of the fiber system to be less rigid and isalso able to compensate for any changes in path length that might occurafter component placement or replacement.

[0077] Optical pulse division is performed by either a waveguide or withsequential optical taps disposed along the length of the optical fiber.In the exemplary embodiment of FIG. 7, ten optical outputs are definedat each level, but this is only for purposes of convenience ofdescription. Any number of optical outputs might be provided for any andall of the different levels of the optical loops depending upon thedesire of the system designer. Further, optical taps may be configuredto pass any percentage of the input optical pulse that is required forsystem performance. That is well understood by those having skill in theart optical taps may be defined that taps 10%, 25%, 50%, 75%, or thelike, of the input optical pulse. Likewise, a waveguide division of anoptical pulse may be of any percentage desired by system requirements.By varying the percentage of a pulse that is transmitted into branchingfibers, it is possible to make the output pulse amplitudes all equal, ormake the output pulse amplitudes different for each branch in the loop.Suitably, a laser pulse maybe split after the laser by means of an OEICwaveguide, a waveguide on a printed circuit board, a waveguide disposedin line with an optical fiber, or by means of individual fiber tapssplitting off a single fiber. The latter option decreases fiber bulk,but it implemented care should be taken to minimize defects in any ofthe fibers or fiber branches. However, an error in measurement of anindividual branches' length (fiber or wave guide) could be compensatedfor by adjusting any available free space gap disposed prior to thephoto detector. Inline amplification, where necessary, maybe providedanywhere within the optical path by a rare-earth doped amplifier (ieEDFA), as is well understood by those having skill in the art. If inlineoptical amplification is required, it would be generally desirable toutilize a 1550 nanometer laser in order to accommodate well understoodamplification methodologies. This is the only foreseeable constraint onlaser wavelength.

[0078] In order to compensate for any possible relativistic effects thatmight occur during the usage, an optical comparator, to be described ingreater detail below, can be disposed in any of the loops, but ispreferably disposed in the initial loop of the system. The opticalcompensator is a device for relatively advancing or delaying an opticalpulse within a set pathway by comparing two light pulses and divertingone or the other pulse into a delaying or advancing side pathway, as ameans to get the two light pulses into synchronization.

[0079] In an additional aspect, the invention contemplates the use of aside optical pathway to advance or delay an optical pulse. A methodologyfor comparing two light pulses and diverting one or the other of themvia optical switches (electrical or optical triggered) into a delayingor advancing side pathway defines a means to get the two light pulses insync with one another. A methodology for changing the relative position(phase) of a particular light pulse with respect to that of anotherlight pulse by using optical switches to divert said light pulse intodelaying and advancing side pathways is also described.

[0080] In an optical timing device, as described above, it is relativelyimportant to compensate for any phase differences developed between aprevious pulse and a newly generated pulse. In this regard, an opticalcompensator (OC) defines a device that can be used within an opticaltiming generator, or any device which needs to advance or delay a lightpulse, or to compensate for any internal or relativistic effects thatmay occur within a device during the transmission of a light pulsewithin a pathway. Optical compensators are used to adjust the relativephase of a new light pulse within the transmission pathway so that it isin sync with a previously generated light pulse or to just advance ordelay a light pulse within the transmission pathway.

[0081] The optical compensator device suitably comprises a side path ora set of side pathways where the light pulse is diverted in order toadvance or delay its time characteristic (phase) relative to the mainpathway. Because each side path is defined in accordance with aparticular distance metric, it will be understood that an advancing pathwill necessarily be shorter than the main pathway. Likewise, a retardingpath will suitably comprise a longer distance than the main pathway.Travel along side pathways is implemented by optical switches whichdivert the light pulse into said side pathway when the optical switchesare turned on. Characteristically, optical switches are simply logicgates that define an optical pathway into one branch (waveguide orfiber) upon occurrence of a particular logical state and define anoptical pathway into another branch upon occurrence of a second logicalstate or condition. Optical switching into such pathways can also bemade to de-synchronize pulses or to further advance or delay aparticular pulse relative to another by changing the logical gatingmechanism being used, as will be well understood in the art of logicalintegrated circuit design.

[0082] The individual pathways can be implemented either as a waveguidepathway or an optical fiber pathway, as described previously. Further,optical switches can be of any make that can meet the speed requirementsnecessary for the device. The optical switches can either beelectronically or optically triggered and are contemplated as beingall-optical (optical core), optical-electronic-optical (o-e-oconfigured), optical-mechanical-optical, and particularly implemented assoliton switches. Each of these component types are well known to thosehaving skill in the art and further discussion of their construction andimplementation would not be particularly germane.

[0083] A simplified schematic for a delay-type optical compensator, inits most basic form, is given in the exemplary embodiment of FIG. 8A,while a simplified schematic for an advance-type optical compensator, inits most basic form, is given in the exemplary embodiment of FIG. 8B. Inboth exemplary embodiments, a sidepath 100 is coupled to a main opticalpathway 102 by an optical switch 104. As a pulse traverses the system,and it is determined that its phase must be adjusted, the optical switchdiverts the pulse either onto an advancing path, a delaying path, orallows the pulse to traverse the main pathway, if desired. Phaseadjustment comes into play, for example, if there were a repeatablephase distortion introduced by the pulse regeneration/laser retriggeringcircuitry. Necessarily, this would cause more of a phase stretch,compelling a more often use of a phase advance pathway.

[0084] A simplified schematic for an optical compensator, which is ableto either advance or delay an optical pulse such that it is insynchronous relationship with a previous pulse, in the main pathway, isshown in the exemplary embodiment of FIG. 8C. In this embodiment, twooptical switches 106 and 108 define entries into delay path 110 and/oradvance path 112, respectively, relative to the main optical pathway114.

[0085] Further, an optical compensator can be configured to adaptivelyadvance or delay a light pulse such that it is in synchronousrelationship with a previous light pulse. One particular embodiment ofthis feature would allow for compensation for any relativistic,electronic phase shift, or intrinsic mechanical changes that may occursuch that would make a newly generated light pulse be out of sync with apreviously generated one. A simplified schematic of an exemplaryembodiment of such an adaptive phase compensator is shown in FIG. 9.

[0086] Specifically, optical switches 116 and 118 are controlled bysimple logic gates (electronic or optical) that appropriately turn onthe correct switch in order to adaptively adjust the phase of a lightpulse. The desired action is to turn on optical switch 118 if the newpulse is ahead of the previous one; the switch 118 remains in the offstate for all other conditions, while the desired action for opticalswitch 116 would be to turn on if the previous pulse is ahead of the newone. Further, operation of each of the switches is adaptive in thattheir operational state is controlled only by the arrival times of boththe previous and the new pulse, and not by any external constraints. Onerelatively simple methodology for accomplishing this feature is to havetrigger edges (rising or falling edges) of each of the light pulses atissue condition a set of simple logic gates, the Boolean arguments ofwhich define the required function. In the exemplary embodiment of FIG.10, a simplified gating diagram for this feature describes an advanceand delay switch implemented using a combination NOT-AND gate setup,with inverted input conditions defining whether the gate is operativelyresponsive as an advance gate or a delay gate.

[0087] The rise time for the optical switch is required to be less thanthe time it would take the new pulse to arrive via the main pathway. Thefall time for the optical switch would also require to be faster thanthe loop time of the main pathway and slower than the greatest possiblediscrepency that could occur between the new pulse and the previous one.Since the advance and delay pathways are of fixed length, multiplesequential optical compensators could be used with each one having ashorter pathway until they are in sync within the acceptable margin oferror (which would typically be less than the pulse width of the lightpulse). This can be relatively easily implemented with a switch fabriccore configured optical switch (a 1×N configuration).

[0088] The optical timing device (lightclock), in accord with theinvention is also able to operate as a pulse regenerating closed loopsystem. The optoelectronic timing device is configured as an OEICcomprising an integrated circuit chip including either a superimposedwaveguide or attached optical fibers as the optical pulse propagationpath. The lightclock is contemplated as having multiple operationalcharacteristics and is constructed with multitiered loops and/or withmultiple lasers: laser(s) for each tier and/or lasers simultaneouslybeing fired for a higher output.

[0089] The optical timing device can be used as simple pulse (optical orelectrical) output device with a frequency in the MHz or GHz range, orhigher as is desired by system timing requirements. The optical timingdevice is further designed to provide an “intelligent” clockingfunctionality with a designed patterned output. The patterned output iscontemplated as comprising any desired interval between the individualpulse's as well as contemplating a predefined variable set of outputamplitude characteristics, such that the periodic occurrence of one of amultiplicity of different amplitude pulses defines an internallyrecursive set of timing triggers. For example, it is well within thecontemplation of the present invention to provide an optical timingdevice that is able to define a 500 MHz timing signal characterized by300 mV pulse amplitudes while the same pulse train defines a 1 GHztiming signal characterized by 200 mV pulse amplitudes. The 200 mVpulses are interspersed within and between the 300 mV pulses to define amodulated pulse train.

[0090] Returning momentarily to the exemplary embodiment of FIG. 6, itcan be easily seen that the secondary loop may be devised with aphotodetector and ancillary electronics configured to output 200 mVpulses, while the primary loop is configured to output 300 mV pulses.Accordingly, the system can be adapted to output a combination 1 GHz and10 GHz signal, with the 10 GHz signal internally dependant on thecharacteristics of the 1 GHz loop.

[0091] As described in various other previous exemplary embodiments, theoptical timing device can be implemented as either an open or closedloop device. The particular configuration discussed in connection withthe exemplary embodiment consists of a single loop or a tiered loopsystem with a regenerated pulse between each loop. The output can bephotonic into a waveguide, optical fiber, or collimated into free space.The output can also be electrical via the addition of a photodetector.In order to provide for an optical pulse having a desirable high pulseamplitude, multiple lasers can be ganged and simultaneously fired, asingle laser (or multiple lasers) may be operated at or above its STthreshold, or a single laser (or multiple lasers) may have its outputamplified in conventional fashion. If multiple lasers are usedsimultaneously then each could be provided with its own initialwaveguide pathway or multiple lasers could be merged via fiber splicingor waveguide pathway convergence.

[0092] As described in connection with the embodiments of FIGS. 5 and 6,if each additional elemental pathway provides an additional travel timeof a known amount (an additional 0.2 ns, for example) and the primaryloop defined a ins cycle then this particular device would exhibit a 5GHz output. A suitable photodetector/pulse regenerator (PD-R) could beas simple as a photoconductive switch with a capacitor or an opticallytriggered FET. An initiator (I) could be as simple as a capacitortrigger to initiate a laser pulse and start the timing cycle.Termination of the closed loop cycling could be achieved by breaking thecircuit anywhere between the PD and the laser.

[0093] In the multiple laser case, a ganged laser set-up issimultaneously fired, but each laser has its optical pulse directed intoa waveguide or fiber having a different length. Thus, a single triggerevent causes pulse with multiple arrival times by virtue of thedifferent travel distances. By allowing the different travel paths to bemanufactured with different attenuation characteristics, each of thearriving pulses may be characterized by unique pulse amplitudes and evenunique pulse widths. By varying the size and the alignment of awaveguide pathway or the splicing percentage of an optical fiber system,different voltage/wattages (electrical/optical output respectively) canbe obtained for each discrete pulse in the loop cycle if desired. Alsoby changing distance between different pulses, the intervals betweeneach pulse in the loop cycle can be different. Various other internalmodulation schemes can also be accommodated as desired by the systemdesigner using well known optical and/or electronic modulationmethodologies.

[0094] Those having skill in the art will immediately understand thatseveral changes and modifications may be made to the embodimentsdisclosed and described without departing from the scope and spirit ofthe present invention. The illustrated embodiments allow for a simpleconceptual understanding to be had of the present invention and it willbe understood that the invention is quite adaptable to numerousrearrangements, modifications and alterations. Accordingly the inventionis not to be limited to the specifics of the illustrated and describedexemplary embodiments, but is rather to be defined only with respect tothe full scope of the appended claims.

1. In an optoelectronic timing system, an adaptive frequency generatorsystem comprising: at least one semiconductor laser configured to issuesubnanosecond optical pulses defining a periodic pulse train; at least afirst optical waveguide, the waveguide configured to define a firsttime-quantifiable optical path for a pulse of the train; at least oneadditional optical waveguide, the additional waveguide configured todefine a second time-quantifiable optical path for a pulse of the traindifferent from the first waveguide; a first nodal point coupled to thefirst and second waveguides at which pulses of the train are directedinto the first and second waveguides; a second nodal point coupled tothe first and second waveguides at which pulses directed into the firstand second waveguides are recombined; and wherein, the length of thesecond time-quantifiable optical path has a defined numericalrelationship to the length of the first time-quantifiable optical path,such that the periodicity of pulses recombined at the second nodal pointhas the same numerical relationship with the periodicity of the issuedpulse train.
 2. The system according to claim 1, wherein the at leastone semiconductor laser is configured to provide a pulsed output havinga periodicity in the range of about 1 nanosecond so as to define a 1gigahertz pulse train.
 3. The system according to claim 2, wherein thesecond optical time-quantifiable optical path has a length differingfrom the first time-quantifiable optical path by about 0.5 nanoseconds,so as to define a 2 gigahertz pulse train at the second nodal point. 4.The system according to claim 1, further comprising: a multiplicity ofadditional optical waveguides each coupled to the first and second nodalpoints, the additional waveguides configured to define a multiplicity oftime-quantifiable optical paths; and wherein, the lengths of each of themultiplicity of additional time-quantifiable optical paths having anumerical relationship with each other and with the firsttime-quantifiable optical path.
 5. The system according to claim 4,wherein the semiconductor laser is configured to provide a pulsed outputat a first periodicity and wherein the recombined pulse train at thesecond nodal point provides a pulse train having a second periodicity,the second periodicity being a multiple of the first, the multipledefined by the numerical relationship between the multiplicity ofadditional time-quantifiable optical paths and the firsttime-quantifiable optical path.
 6. The system according to claim 5,wherein the semiconductor laser operates at a frequency of about 1gigahertz.
 7. The system according to claim 6, wherein the lengths ofthe multiplicity of time-quantifiable optical paths differ from oneanother by about 0.2 nanoseconds, so as to define a 5 gigahertz pulsetrain at the second nodal point.
 8. The system according to claim 7,wherein time quantification of the optical path length is defined by thedistance required for a pulse to travel at the speed of light for agiven time interval.
 9. In an optoelectronic timing system, a method foradaptive frequency generation, comprising: providing at least onesemiconductor laser configured to issue subnanosecond optical pulsesdefining a periodic pulse train; providing at least a first opticalwaveguide, the waveguide configured to define a first time-quantifiableoptical path for a pulse of the train; providing at least one additionaloptical waveguide, the additional waveguide configured to define asecond time-quantifiable optical path for a pulse of the train differentfrom the first waveguide; directing pulses of the train into the firstand second waveguides at a first nodal point; recombining pulses of thetrain from the first and second waveguides at a second nodal point; andconfiguring the lengths of the first and second time-quantifiableoptical paths to have a defined numerical relationship to one another,such that the periodicity of pulses recombined at the second nodal pointhas the same numerical relationship with the periodicity of the issuedpulse train.
 10. The method according to claim 9, wherein the at leastone semiconductor laser is configured to provide a pulsed output havinga periodicity in the range of about 1 nanosecond so as to define a 1gigahertz pulse train.
 11. The method according to claim 10, wherein thesecond optical time-quantifiable optical path has a length differingfrom the first time-quantifiable optical path by about 0.5 nanoseconds,so as to define a 2 gigahertz pulse train at the second nodal point. 12.The method according to claim 9, further comprising: defining amultiplicity of additional optical waveguides each coupled to the firstand second nodal points; configuring the additional waveguides to definea multiplicity of time-quantifiable optical paths; and wherein, thelengths of each of the multiplicity of additional time-quantifiableoptical paths having a numerical relationship with each other and withthe first time-quantifiable optical path.
 13. The method according toclaim 12, wherein the semiconductor laser is configured to provide apulsed output at a first periodicity and wherein the recombined pulsetrain at the second nodal point provides a pulse train having a secondperiodicity, the second periodicity being a multiple of the first, themultiple defined by the numerical relationship between the multiplicityof additional time-quantifiable optical paths and the firsttime-quantifiable optical path.
 14. The method according to claim 13,wherein the semiconductor laser operates at a frequency of about 1gigahertz.
 15. The method according to claim 14, wherein the lengths ofthe multiplicity of time-quantifiable optical paths differ from oneanother by about 0.2 nanoseconds, so as to define a 5 gigahertz pulsetrain at the second nodal point.
 16. The system according to claim 15,wherein time quantification of the optical path length is defined by thedistance required for a pulse to travel at the speed of light for agiven time interval.
 17. In an optoelectronic timing system, an adaptivefrequency generator system comprising: at least one semiconductor laserconfigured to issue subnanosecond optical pulses defining a periodicpulse train; a multiplicity of optical waveguides, the waveguidesconfigured to have physical lengths differing from one another by anumerical relationship, each length defining a time-quantifiable opticalpath for a pulse of the train based upon the time required for a pulseto travel a particular length at the speed of light; a first nodal pointcoupled to the multiplicity of optical waveguides at which pulses of thetrain are directed into the multiplicity of optical waveguides; a secondnodal point coupled to the multiplicity of optical waveguides at whichpulses directed into the multiplicity of optical waveguides arerecombined; and wherein, the periodicity of pulses recombined at thesecond nodal point has the same numerical relationship with theperiodicity of the issued pulse train as the numerical relationship ofthe multiplicity of optical waveguides.
 18. The system according toclaim 17, further comprising: a pulse detector; a regenerator coupled tothe pulse detector and semiconductor laser; a regeneration waveguidehaving a length equal to the longest length of the multiplicity andcoupled to receive pulses from the laser, the regeneration waveguide notcoupled to the first or second nodal points; and wherein, a pulsetraveling the regeneration waveguide directed to the pulse detector andregenerator so as to trigger the laser to issue a next pulse, thephysical length of the regeneration waveguide defining a fundamentalfrequency of the system.
 19. The system according to claim 18, whereinthe fundamental frequency of the system is an integer.
 20. The systemaccording to claim 19, wherein the periodicity of pulses recombined atthe second nodal point defines a frequency which is a multiple of thefundamental frequency of the system, the numerical value of the multiplebeing equal to the number of the multiplicity of optical waveguides.